Vertical positioning of transistor on heatsink

[Eric]

Hi Everyone,

It seems that I'm the only one building a Pass Labs a40 around here, but hoping that some of you might have some insights to share with me.

The a40 has a total of 8 output transistors, for which I have purchased 8 Wakefield 423 heatsinks. The heatsinks are 5.5" tall and have three mounting positions: one near the top, one vertically centered on the heatsink, and one near the bottom of the sink.

My question is which location is best to place the transistor: in the middle, or in the bottom position? It seems that the middle position might get better heat distribution, but the bottom position might benefit most due to convection.

Does anyone have some insights to offer?

Thanks!
Eric

 

[vpharris]

Probably in the middle. Actually it seems that they should be ideally mounted slightly offset toward the top. Look at the nice plot done by Wayne Sankey. You can use the software he used by getting it at the R-Theta website if you want to do the analysis.
http://www.passdiy.com/images/gallery/a2-p1-f3.gif

 

[Eric]

Thanks, Vince! I had forgotten about Wayne's thermal simulation. Looks like I'll be using the center mounting position...

Eric